1. Field of the Invention
This invention relates generally to semiconductor processing and, more particularly, to pressure control systems for process chambers.
2. Description of the Related Art
Many semiconductor fabrication processes, or processes for fabricating integrated circuits, require that the pressure within a process chamber be varied between relatively high and relatively low pressures. For example, a chamber might be at, e.g., atmospheric pressure, for loading a semiconductor substrate into the chamber and then the chamber is pumped down or evacuated to a relatively low pressure, e.g., a vacuum, for processing of the substrate. When pumping down a process chamber from the relatively high pressure to the relatively low pressure, the pump-down rate is preferably not so large as to cause undesirable turbulence, which can stir up particles within the process chamber or within the piping network attached to the process chamber. Such turbulence or stirring up of particles is undesirable because these events can contaminate or otherwise adversely affect the quality of the process results on the substrates.
To avoid such undesirable turbulence, a process chamber pump-down typically occurs in two-steps. First, the pump-down is started through a small orifice valve, a so-called “slow-pump valve,” which, because of the small size of the valve's opening, limits the rate at which gas can exit the chamber. This aids in minimizing both gas turbulence and the stirring up of particles. After some time pumping through the slow pump valve or after reaching a particular chamber pressure, a large opening valve, or so-called “fast-pump valve,” is opened to complete the pump-down to the low processing pressures.
In addition to preventing the flow of exiting gas from being too large during pump-down, another pump-down requirement is that the pump-down time is preferably as short as possible, to allow for a maximum utilization of the chamber for processing. These are, however, conflicting requirements.
A vacuum pump system for a two step pump-down according to the prior art is shown in FIG. 1. The chamber pressure 10 can be measured by three different baratrons, or pressure sensors, 12, 14, and 16. The pressure sensor 16 is used for measuring and controlling the pressure of the chamber 10 during processing and is designed to measure pressures in a range of 0 to 2 Torr. The pressure sensor 12 is designed to measure pressures in a range of 0 to 1000 Torr and is used to determine if the chamber has reached atmospheric pressure and if the chamber can be opened to load or unload substrates. Optionally, a third pressure sensor 14 can also be used to more accurately measure intermediate pressures in the range between the pressures measured by sensors 16 and 12, for, e.g., processing at those intermediate pressures. The sensitive pressure sensor 16 can be provided with a valve to avoid exposure of the valve to high pressures. In that case, the pressure sensor 14 is used to detect if the pressure is below a predetermined level (e.g., 5 Torr or 10 Torr) so that the valve that brings pressure sensor 16 in communication with chamber 10 can be opened and so that the valve can be closed when the chamber pressure increases above the predetermined level. The pressure of the chamber 12 can be controlled using an optional butterfly valve or other variable opening valve or by using a variable N2 gas ballast, all indicated by reference numeral 18. A pressure sensor 20 at the pump inlet 22 is typically used for safety purposes only, to detect unsafe pressure levels. Typically, this sensor 20 is designed to measure pressures in a range of 0 to 10 Torr.
The pump-down of the chamber 10 proceeds in an uncontrolled manner, as described above; that is, the rate of mass flow of gas out of the chamber 10 can vary continuously over the course of a pump down. Typically, a slow pump valve 24 is first opened. Then, after the chamber 10 has reached a particular pressure, a fast-pump valve 26 is opened to complete the pump-down of the chamber 12.
U.S. Pat. No. 6,139,640 proposes a method for pumping-down a process chamber in a more controlled manner, to decrease the amount of time needed to evacuate a vacuum chamber. That patent proposes providing a pump system with a mass flow controller (MFC) in a vacuum line. The process chamber pump-down is regulated using the mass flow controller to control the exiting gas flow at a constant value. Mass flow controllers, however, are additional and relatively complex and expensive components. Undesirably, their use increases both the cost and the complexity of processing systems. Further, mass flow controllers are susceptible to contamination and their use in a pump-down line can easily become problematic.
Accordingly, there exists a continuing need for systems and methods of pumping down a process chamber while minimizing gas turbulence and the stirring up of particulate matters.